It is a common problem to park or otherwise position a vehicle accurately within the limited confines of a typical residential garage. Accurate positioning is necessary to prevent the front of the vehicle from hitting the end wall, or objects placed on or along the end wall, as the vehicle is being driven into the garage. It is also often necessary to have adequate clearance in front of the vehicle for walking between the end wall and the vehicle while retaining some clearance at the rear of the vehicle so the overhead garage door can close without hitting the vehicle. It is inconvenient, at a minimum, to have to exit the vehicle to check clearances before closing the overhead garage door, and can be a potentially serious security risk when the garage is located in an area that is susceptible to crime.
A similar positioning problem exists when backing boats, camping trailers, and other recreational vehicles into parking stalls or when backing semi-tractor trailers and other industrial vehicles up to loading docks. In each case, it is necessary to position the trailered vehicle close to, while avoiding contact with, adjacent objects. In yet another application, it is necessary for forklift operators to place palletized loads close to walls and other obstacles without coming into contact with them.
Some devices are known which assist the operator in parking or positioning a vehicle. These devices are generally of a simple mechanical type that require the vehicle to contact a mechanical element, such as an arm, a lever, a switch, a post, a ball, etc., which then can be used to actuate an alarm. Devices of this type, while effective in the positioning of vehicles, are cumbersome and awkward, often invading floor and walking space around the vehicle. In addition, they are difficult to adjust, requiring tools, and prone to tampering by children because of their close proximity to the vehicle.
U.S. Pat. No. 4,808,997 to Barkley discloses a ceiling mounted device that emits a downward beam of light, a portion of which is reflected by the floor back to the device, producing a corresponding electrical signal. When a vehicle intercepts the beam, it theoretically produces a substantial change in the amount of reflected light to the device resulting in an electrical signal change, thus triggering an alarm device. In practice, however, variables including the reflective qualities of the vehicle, such as the surface finish condition and the angle of the vehicle surface which intercepts the light beam, may result in an insufficient change in the amount of reflected light necessary to trigger the alarm.